Throughout history man has controlled the water activity of food through drying, addition of salt or sugar, and freezing such that the food becomes stable to microbial and chemical deterioration. These methods prevent spoilage and maintain food quality.
Water activity is a measure of how efficiently the water present can take part in a chemical (physical) reaction. If half the water is so tightly bound to a protein molecule that it could not take part in a hydrolysis reaction the overall water activity would be reduced.
The water activity of a food describes the energy state of water in the food, and hence it's potential to act as a solvent and participate in chemical/biochemical reactions and growth of microorganisms.
Water activity, not water content, determines the lower limit of available water for microbial growth. The lowest aw at which the vast majority of food spoilage bacteria will grow is about 0.90.
The free or available water in a food supports microbial growth, and participates in and supports chemical and enzymatic reactions and spoilage processes. It is this amount of free water which is called water activity, aw, and it is more important for food stability, chemical and microbial, than total water content.
It is an important property that is used to predict the stability and safety of food with respect to microbial growth, rates of deteriorative reactions and chemical/physical properties.
Water activity: Roles in food safety and food quality
